Abstract: A785MPa-grade weldable HSLA steel (HQ80C) with Cr, Ni, Mo, V, Ti, and B has been experimentally studied to determine quantitative relationships between properties (strength and toughness) and microstructures. It has been found that the ductile-brittle transition temperature (50%FATT) varies linearly with the mean intercept of martensitic packets. An empiric equation has been established to predict the yield strength of the as quenched and tempered steel; the relative deviation of the predicted yield strength values from the experimental ones are all less than 3% when the steel was tempred at 500 to 700℃.